Philae finds organic molecules on comet: what’s the big deal?

Everyone is excited that organic molecules have been detected by the probe Philae on the comet 67P/Churyumov-Gerasimenko.  This, people say, could explain the origin of life on Earth: the planet was seeded by the carbon-containing molecules on comets, and those organic seeds help create the first replicators that eventually became things that were indubitably “alive.”

But wait: we already had carbon-containing molecules on Earth, and we have no idea what molecules Philae found. The cometary compounds could, for example, be methane (a simple molecule with one carbon and four hydrogen atoms)—a molecule unlikely to have played a major role in the origin of life. Ditto for carbon monoxide or carbon dioxide (CO and CO2 respectively). The early Earth already had carbon in those molecular forms in the atmosphere, so why did we need to get them from a comet or asteroid? There were also physical forces on Earth, like heat and pressure around thermal vents, that could synthesize more complex organic molecules like amino acids. We also know that peptides (small proteins composed of amino acids) might have formed under early-Earth conditions.

Given that the constituents of early life could have formed under early Earth conditions (granted, as Matthew mentioned yesterday, we have little idea of how it happened), why invoke life being helped along by molecules on comets, molecules that haven’t even been identified yet?

I’ll be more impressed if they find complex amino acids or—even better but even more unlikely—proteins on the comet.  But until they do, at present I would echo Laplace and say that there’s no real need for a “cometary theory of abiogenesis.” Did the comet have geophysical conditions, or come from some planet with those conditions, that were even more favorable to the formation of complex organic molecules than the conditions on Earth?

What I’m saying, then, is that all the heated speculation that life on Earth might have been catalyzed by stuff on comets like  is a very premature speculation. Let’s wait and see what they found on comet 67P.

h/t: Melissa

71 Comments

  1. Posted November 19, 2014 at 8:55 am | Permalink

    Sub

  2. Joseph Stans
    Posted November 19, 2014 at 9:02 am | Permalink

    Much more impressive if they found a six inch tardigrade with a Galactic Association of Realtors pin …

  3. Posted November 19, 2014 at 9:04 am | Permalink

    Seems people have a predisposition towards needing an explanation of origins that comes from without rather than within. I’m curious whether anyone has investigated the biological imperative for that need.

    • Scientifik
      Posted November 19, 2014 at 9:38 am | Permalink

      It’s easy to see why the idea captures people’s imagination 🙂

      http://www.popsci.com/article/science/see-comet-tails-flap-solar-wind

    • Hempenstein
      Posted November 19, 2014 at 9:44 am | Permalink

      Seems people have a predisposition towards needing an explanation of origins that comes from without rather than within.

      Exactly my thoughts, and figured someone had already said it. In the beginning, there was nothing here – it hadda come from somewhere else to start.

      • Posted November 19, 2014 at 10:01 am | Permalink

        Isn’t Earth and all the other bodies in the Solar System just ejecta from our very average sun? I think that’s what Carl Sagan told me, we’re all “star stuff.” Why wouldn’t the planet in the just-right Goldilocks position have a just-right physical composition? I thought the comet water theory had more support than it seems to have, but I have never bought the idea that life is extraterrestrial: how would the building blocks have gotten onto the comet? It had to be generated somewhere; why not here?

        • Posted November 19, 2014 at 10:38 am | Permalink

          The building blocks may as well have been formed on earth, or they rained down from space. Or both.
          One of the things to look for in comets is if their is a ‘fingerprint’ of isotopes in its various atoms, and if a similar fingerprint of isotopes are found on earth. Isotopes of oxygen, for example. If they are similar, then that could mean that a lot of our water came from comets.
          Or maybe not. Anyway, I want to find out.

          • Posted November 19, 2014 at 10:39 am | Permalink

            Well,I see bonetired right below me has dashed that idea. Ok then.

        • Posted November 19, 2014 at 10:50 am | Permalink

          Short version: by roughly several seconds “after” the Big Bang (notions of time get fuzzy as well as what event the “Big Bang” itself refers to, but this is “long” after said fuzziness), nearly all the atomic nuclei of the early Universe had formed, overwhelmingly hydrogen with a bit of helium and an insignificant smattering of other elements.

          A few hundred thousand years later, things would have cooled enough for some of that matter to have begun to start coalescing into clouds which further collapsed into stars; we see the remnants of the transition to this period in the sky today as the Cosmic Microwave Background.

          Those first stars were mostly hydrogen with a big of helium; no significant amounts of new elements had formed since immediately following the Big Bang. The fusion in those stars would mostly have created more helium, but some (possibly most?) of those stars would have been big enough to live short lives that ended in supernovae explosions, and those events both would have created all sorts of heavier elements and scattered most of what was in the star at that point (still mostly hydrogen) far and wide.

          That scattered matter would itself have eventually coalesced into more clouds that again eventually collapsed into stars, but these stars would have had slightly richer chemistry that would have led to even more richness in the elements manufactured if and when they turned supernova.

          Our star formed after at several generations of that sort of thing. Basically everything in the Solar System today would have been part of the same interstellar molecular cloud from which the Sun coalesced…but, of course, plain randomness plus various other dynamics would have made things “lumpy” with uneven distributions.

          The dust that coalesced to form the Earth would initially have been indistinguishable from the same dust that coalesced to form the other planets and the comets and asteroids and the Sun, save, of course, for whatever lumpiness was there.

          …and, at this point, you have an ancient conundrum. If you place a single grain of sand on a table, you obviously don’t have a pile of sand. Add a second grain of sand, you still don’t have a pile. But, if you keep adding sand a single grain at a time, eventually you will have something that is unquestionably a pile. But which grain of sand was it that turned it from grains of sand on the table to a pile?

          Similarly, by at least four billion years ago, piles of grains of sand had coalesced into our planet and the others. At that point, you can definitely meaningfully describe additional infalling material from comets as foreign…but it wouldn’t have been all that much earlier that the Earth was just a somewhat larger lump of sand than the other surrounding lumps, and not long before that all there was was just sand.

          …so, in one sense, the question doesn’t make sense at all.

          In another sense, the interesting bit is to try to suss out the distribution of matter of the early Solar System. Was there something about organic matter that would have caused it to distribute disproportionally in an outer region such that only after planetary formation would it have migrated inward? If so, that’d be really interesting.

          Sorry…I guess that wasn’t very short, after all, even if it probably still is the “short version.”

          b&

          • Posted November 19, 2014 at 11:24 am | Permalink

            I think it depends on the geometry of the sand grains. Different angles of repose mean that x grains of one mix of sand might form a pile, whereas another might require 2x grains, or only .5x grains, to form something that anyone would call a pile. I don’t know if other surface properties or dust or ambient moisture would have any effect, but … wait … I’m sorry; what were we talking about … ?

          • Posted November 19, 2014 at 11:39 am | Permalink

            Oh, right: organic matter. So I guess there is some conflation in people’s minds of the terms organic and life. At what point does a pile of carbon, phosphate, calcium, some metals, and water constitute life? Never: life is not composition, it’s process. So yes anything we learn about how the elements organized into the various clouds and bodies is fascinating and significant, but it’s process that generated, shaped and sustains life. And those processes are terrestrial whether the raw materials were in place from “the beginning” or were deposited subsequently.

            • Posted November 19, 2014 at 11:48 am | Permalink

              You’ve probably nailed it. To chemists, “organic” simply means, “carbon.” But, to most everybody else, “organic” means, “wholesome living things you’d find in the expensive sections of the grocery store.”

              If the reporting wasn’t about “organic molecules” but rather “carboniferous deposits,” we wouldn’t be having this discussion.

              …of course, in that case, the discussion would instead be about the merits of building a pipeline to a comet to extract its shale oil. And it’d be about as coherent as the one we’re having now….

              b&

              • Garnetstar
                Posted November 20, 2014 at 7:50 am | Permalink

                Yes. The terms “organic” and “inorganic” arose from back in the day when chemists also thought there was something special about life, and that the chemicals involved had some mystic essence.

                There was immense uneasiness when it was found that “organic, living” chemicals could be synthesized from “inorganic,non-living”, and functioned the same in the body.
                Synthetic thyroid hormone was one of the first. What does that say about humans: you mean we’re not special, infused with life force, just made of chemicals? Yes.

                This was only back in the 1930’s, not so long ago.

        • Posted November 19, 2014 at 2:30 pm | Permalink

          “why not here?”

          Exactly, why is a comet a more likely place for life to have started?
          A finding that “those organic seeds” arrived on earth via a comet instead of developing on earth doesn’t speak too much to the origin of life either, it just places it in a different venue. It’s a neat sci-fi type of theory but it leaves fundamental questions about the origin of life unanswered.

          • Torbjörn Larsson, OM
            Posted November 19, 2014 at 3:55 pm | Permalink

            Asteroids and comets have been fairly well excluded as habitable. When they are heated early during initial aggregation, the freezing liquid zone travels much too fast for emergence.

        • Torbjörn Larsson, OM
          Posted November 19, 2014 at 3:17 pm | Permalink

          No, it is the other way around. When a molecular cloud collapse under gravity and friction it forms a star- and planet-forming disk.

          The “ice line”, the area where water could remain without being vaporized by the early star irradiation (IR, UV and X ray), were somewhere out at the asteroid belt at the time.

          But see my longish comment how the disk is now thought to behave, the water was likely rapidly spread and encapsulated within the aggregating planetesimals.

  4. bonetired
    Posted November 19, 2014 at 9:06 am | Permalink

    Just been announced that the deuterium-to-hydrogen of water in the very tenuous atmosphere of the comet is much higher than that found on Earth. That seems to cast further doubt on the hypothesis that Earth’s water came from comets. I would also guess that if water on Earth didn’t ( at least primarily) come from comets then neither did any organic material.

    http://news.sciencemag.org/europe/2014/11/doomed-comet-lander-delivered-harvest-science

    • bonetired
      Posted November 19, 2014 at 9:08 am | Permalink

      “deuterium-to-hydrogen ratio” although I suspect that everyone will guess that …

    • Timothy Hughbanks
      Posted November 19, 2014 at 9:21 am | Permalink

      It is not clear to me whether the D:H ratio in the comet now is relevant. How different was the D:H ratio of the galaxy 4.5 billion years ago when the earth formed. Another factor I don’t know anything about (others may): does the age of the comet strongly affect the D:H ratio? Don’t comets lose mass as they age because they lose volatiles when they heat up when their orbits carry them nearer the sun? If so, fractionation of the water would favor retention of the lass volatile D₂O and HDO in the comet over time. I don’t know the order of manitude of that effect, however.

      • Posted November 19, 2014 at 10:11 am | Permalink

        I wonder if mission scientists are happy to have an idea that gets the public’s attention, or maybe the “potential origins of life” bulletpoint is a critical part of mission justification, that it’s not just science for its own sake? Or if it is at all disappointing that understanding comets is not enough for people, that the main impression lay people have is that we are searching for evidence in support of a hypothesis? Or maybe they just tune it out and enjoy the work: I hope it’s the last one.

      • Kevin
        Posted November 19, 2014 at 12:16 pm | Permalink

        The dissociation of H2O is also more energetically favorable than D2O. In the harsh environment of space, integrating over a long time…D2O might be in much higher abundance on a n atmosphere-less object. That should also be fairly straightforward to predict if one assumes the D2O and H2O see the same EM and particles radiation over time.

      • Torbjörn Larsson, OM
        Posted November 19, 2014 at 3:23 pm | Permalink

        Note that D/H ratios alone can’t exclude comets as volatile deliverers, some comets have the same D/H as Earth, Moon, Vesta, Mars. But nitrogen isotope ratios do exclude comets very clearly! [“Early accretion of water in the inner solar system from a carbonaceous chondrite-like source”, Sarafian et al, Science, Oct 2014]

        The problem is that spectroscopic observations of them are seeing the volatiles in the coma and tail, where the isotope ratios could be different from the comet as such due to fractionation. (How the volatiles melt and percolate is mass dependent.) There is wiggle room for “comet supporters”.

        Philae could have ground truthed that. Now perhaps we can get near surface volatiles from the surface dust that are seen as somewhat truer at best.

    • Nick
      Posted November 19, 2014 at 3:27 pm | Permalink

      Thanks bonetired. This is the article I have been hoping for that will have to do until the actual scientific papers start appearing. I was hoping we wouldn’t have to wait 6 months to a year or more for the so-called “science embargo” to be lifted.

  5. NewEnglandBob
    Posted November 19, 2014 at 9:06 am | Permalink

    But did Philae find evidence of Jaysus? Maybe his profile in rock?

  6. Timothy Hughbanks
    Posted November 19, 2014 at 9:11 am | Permalink

    I completely agree – nothing to get excited about here yet. The average density of the comet is 0.4 g/cm³. What else can there be in the comet but water and other light atom “organic” molecules – and a fair amount of void space. It seems that there is unlikely to be even much carbonate since that would require metal counterions that are heavier (lithium being astronomically rare).

    • Posted November 19, 2014 at 10:16 am | Permalink

      Nothing except the fact that anything got detected at all: they flew a robot a zillion miles and dropped it on a f***king comet! If they get one piece of comprehensible data out of it, whatever it means, it’s still pretty exciting. To me anyway!

    • Posted November 19, 2014 at 10:42 am | Permalink

      I think there should be some metals on it. Not all of the metals in the early solar system would go into forming the rocky bits of planets.

    • Torbjörn Larsson, OM
      Posted November 19, 2014 at 3:25 pm | Permalink

      Asteroids and comets overlap re composition and void fraction. The disk has likely been heavily mixed.

  7. Sastra
    Posted November 19, 2014 at 9:15 am | Permalink

    It seems to me that discovering organic molecules on a comet only falsifies the hypothesis that “Life on earth could not have been seeded by a comet because there are no organic molecules on comets.” It only gets us closer to the truth by ruling something out. It’s not a point against terrestrial abiogenesis.

    • Posted November 19, 2014 at 9:20 am | Permalink

      The problem, Sastra, is that the kind of “organic molecules” on Philae might not have been appropriate for seeding life on Earth. We can’t even address your hypothesis until we know that.

      • Luis
        Posted November 19, 2014 at 9:33 am | Permalink

        I think the fact that they mentioned “organic molecules” probably means they didn’t find just methane (which is already knwon to be present in the nuclei of comets) but found some other compounds as well. I guess we will have to wait to see if these can be identified.

        • Hempenstein
          Posted November 19, 2014 at 9:48 am | Permalink

          I’m going to guess that the very first level of analysis is something like what the “sniffer” that gas companies use to detect leaks. I’m not sure how they work, but my arms-length understanding is that they register anything organic.

          • Hempenstein
            Posted November 19, 2014 at 9:49 am | Permalink

            Probably make that organic and volatile.

      • Posted November 19, 2014 at 10:49 am | Permalink

        Wait and see does seem to be the right approach, but I’m hopeful the results will be exciting. It looks like the COSACS instrument package contains a TOF Mass Spec and gas chromatograph. If they’re claiming evidence of organics, that would suggest that they already have chromatographic and TOF data, and that would mean they already know masses and mobility, right?

        Fingers crossed it’s something that tells us more about the origins of heavy organics.

      • Sastra
        Posted November 19, 2014 at 12:43 pm | Permalink

        Wouldn’t the hypothesis that “there are NO organic molecules on comets (so they could not have seeded earth)” still have been falsified?

        Though as you say the next step is just as significant — figuring out whether they are even the right kind of organic molecules. And if they are, then there are crucial steps continuing on after that one before anyone gets anywhere definitive, if ever.

        The point I was trying to make is that a very basic hypothesis being falsified is really just a technical “victory” which doesn’t start shifting the debate to the ‘comet’ side. All that happened given this scant information is that it was not shut it down … yet.

        Personally, I’m persuaded by your argument that conditions on earth are probably going to turn out to be far superior — and Philae’s discoveries on this front are only going to provide the solid evidence for why.

  8. Kevin
    Posted November 19, 2014 at 9:30 am | Permalink

    I am tempted to scorn public release of this information from this mission as hype*measurement_without_review.

    How many times have major missions to space glorified a banal result? If you count missions to uncover mysteries of radiation in space, then that number is greater than half a dozen in less than a decade.

    High priced science, today, always comes, with the inauspicious duty to manufacture profundity.

    • Posted November 19, 2014 at 9:54 am | Permalink

      Four words: meteoric Martian tubeworm fossils.

    • rickflick
      Posted November 19, 2014 at 9:56 am | Permalink

      Indeed. They must have an office to do promotion which picks up these tidbits and spins them so that the wider, otherwise unappreciative, audience will glance up from their portable device and go – “Huh?”

      • Posted November 19, 2014 at 10:03 am | Permalink

        Arsenic-based life forms: the phrase just writes its own tabloid headline.

  9. Posted November 19, 2014 at 9:56 am | Permalink

    Lots of complex (more complex than methane, for example, over a dozen amino acids, pyrimidines and purines, many other hydrocarbons) have for decades been found in carbonaceous meteorites; see:

    Murchison meteorite
    http://en.wikipedia.org/wiki/Murchison_meteorite

    The Nature and Distribution of the Organic Material in Carbonaceous Chondrites and Interplanetary Dust Particles
    http://www.lpi.usra.edu/books/MESSII/9008.pdf

    Extraterrestrial nucleobases in the Murchison meteorite
    http://astrobiology.gsfc.nasa.gov/analytical/PDF/Martinsetal2008.pdf

    Plus emission and absorption spectra indicate interstellar organics in molecular clouds:
    http://www.space.com/13401-cosmic-star-dust-complex-organic-compounds.html

    • Torbjörn Larsson, OM
      Posted November 19, 2014 at 3:30 pm | Permalink

      Comets do demonstrate the ease of chemical evolution, as you eminently show.

      The question is if that is all they do. (My personal guess.)

  10. Posted November 19, 2014 at 10:05 am | Permalink

    Here is something a geologist can comment on. There are three possible sources of simple organic compounds on the early Earth:

    1. Manufactured in the atmosphere or along oceanic ridges by a variety of inorganic processes.

    2. Delivered to the Earth’s surface via meteorites.

    3. Delivered to the Earth’s surface via comets.

    We have abundant evidence from carbonaceous chondrites that these meteorites at least could have delivered to Earth a host of organic compounds, including aliphatic hydrocarbons, aromatic hydrocarbons, amino acids, carboxylic acids, sulfuric acids, phosphoric acids, alcohols, aldehydes, ketones, sugars, amines, amides, nitrogen heterocycles, and sulfur heterocycles. (I’ll leave it to others to judge the significance of these compounds when it comes to life. Here is a reference: http://web.gps.caltech.edu/classes/ge133/reading/sephton.pdf).

    Comets formed further out in the Solar System and so may have different inventories of organic matter. I think planetary sciences would be shocked if they were not richer in a variety of organic compounds than the carbonaceous chondrite meteorites, but as yet we do not know what they actually contain.

    The question of what comets delivered to the Earth goes beyond organic compounds. I don’t know the precise state of the debate, but there has been an on-going debate as to whether most of the oceans and atmosphere were volcanically outgassed from the interior of the Earth following accretion or whether our volatile inventory was largely captured from impacting comets or from early vapor-rich nebular materials in the earliest days of the Earth.

    So, for me the main question would be the types and concentrations of organic compounds that the comets could have delivered to the Earth. Some delivery of organics seems certain.

    • Torbjörn Larsson, OM
      Posted November 19, 2014 at 3:36 pm | Permalink

      The momentum is right now going to outgassing. The early Earth was “cold” and wet (Jack Hill zircons), and later delivery likely minimal.

      Even nitrogen can be outgassed but likely delayed since the described mechanism is subduction of oceanic crust. [Um, reference. I have to come back on that, my collected list is a mess right now. It’s probably in my “read yesterday” list…]

  11. frankschmidtmissouri
    Posted November 19, 2014 at 10:06 am | Permalink

    The hype is due to Carl Sagan (it always comes down to Carl ;), who promoted the idea that a substantial amount of prebiotic organic material came on comets.

    We already now that the Murchison meteorite contains a mix of amino acids that is close to that produced in Miller-Urey experiments. So extraterrestrial sources of organics can’t be ruled out.

    The nature, concentration and distribution of these organics on 67P would allow a test of the Sagan hypothesis, if not a rigorous one.

    • Torbjörn Larsson, OM
      Posted November 19, 2014 at 3:38 pm | Permalink

      Yes, and he wrote same nice papers on the Cosmos presented “soup” too!

      That was before some of the soup problems was encountered…

    • Posted November 20, 2014 at 10:15 am | Permalink

      Speaking of Carl, I could have sworn that his book with Sklofskii says that organic molecules (methane at least) were already known in comets, and that’s from the 1960s.

  12. Posted November 19, 2014 at 10:34 am | Permalink

    Methane and formaldehyde, etc., would not be a big deal to scientists, but such a finding provides a valuable ‘teaching moment’ to laypersons. They are now paying attention, for a moment, and here is our chance to make some noise about how common organic molecules are.
    Now, finding amino acids or sugars would also not be unexpected since these have been found in meteors. But a comet — this comet in particular– is held in a fairly pristine state since the beginning of our solar system. What would be important to learn is if any amino acids and sugars are biased in their left- or right-handedness. Biotic amino acids are all left handed, and biotic sugars are all right handed. Why that is is one of the lingering mysteries about the origin of life. But IF such things were found on a comet, with those biases in handedness, then that could mean that molecules from space made a significant contribution to providing the first building blocks for life.

    • Posted November 19, 2014 at 10:54 am | Permalink

      There are D/L enantiomeric excesses (>7% in some cases) in the chiral species of amino acids and other optically active organic compounds found in the Murchison meteorite. These enantiomeric excesses could be the result of asymmetric differential destruction in interstellar molecular clouds of enantiomers by ultraviolet light from nearby stars that has been circularly polarized by passing through dust clouds surrounding bright stars. See the SciAM article at:
      http://www.americanscientist.org/issues/feature/polarized-starlight-and-the-handedness-of-life

      • bonetired
        Posted November 19, 2014 at 11:33 am | Permalink

        Urghhh .. Subscription required. Can you give a bit more information, especially why a certain enantiomer (L for amino acids) is prefered?

      • Torbjörn Larsson, OM
        Posted November 19, 2014 at 3:44 pm | Permalink

        Yes, soup theories needed that. (But I believe today amplification mechanisms that can in principle pick up on random chiral excess are known, with clay compartments and what not.)

        If you go to theories of local organic production in geophysical systems (mainly hydrothermal vents), they are pretty much non-chiral until evolution breaks it. (Latest at the evolution of the genetic code, presumably.)

        The latest is that the RNA world can survive racemic metabolism, they have discovered cross-chiral polymerase ribozymes. Short and generic replicating, too, better than uni-chiral polymerases.

  13. Gerorge Martin
    Posted November 19, 2014 at 10:50 am | Permalink

    Indeed, as Jerry’s title says: “What’s the big deal?” It would have been very remarkable if they hadn’t found any. Organic molecules are common in interstellar space and the number found include simple sugars and amino acids. You can find a list of what’s been found here in wikipedia: http://en.wikipedia.org/wiki/List_of_interstellar_and_circumstellar_molecules

    George

  14. ladyatheist
    Posted November 19, 2014 at 11:45 am | Permalink

    If comets have organic molecules, then perhaps the Earth isn’t such a privileged planet. Perhaps other planets somewhere could have life based on Philae’s raw material.

    If they can move the goalposts, why can’t we?

    • Torbjörn Larsson, OM
      Posted November 19, 2014 at 3:47 pm | Permalink

      Good idea, but it turns out that it is better if we can exclude comets. Today it is feasible, even supported by earliest data, that water and other volatiles were present during planet formation.

      That means more systems have habitable planets, and that they can be inhabited for a longer time than if comets would deliver water late.

  15. Posted November 19, 2014 at 12:09 pm | Permalink

    Reblogged this on Reasonable Rants.

  16. Posted November 19, 2014 at 1:29 pm | Permalink

    Reblogged this on peakmemory and commented:
    Panspermia? Maybe not.

  17. Posted November 19, 2014 at 2:43 pm | Permalink

    Dumb Guy Question:

    Don’t comets spend their entire existence orbiting one star? They don’t hop orbits form one star to another, do they?
    If this is true then doesn’t that mean that all the material comprising 67P is from this solar system.
    Doesn’t that mean that earth and 67P were, for lack of a better term, cooked in the same solar kitchen?

    • Posted November 19, 2014 at 2:46 pm | Permalink

      There are certainly rare exceptions of interstellar interlopers, but they’re certainly rare. I’m sure we haven’t actually found any, but, statistically, they must exist.

      That caveat aside, you’re exactly right.

      b&

    • Torbjörn Larsson, OM
      Posted November 19, 2014 at 3:51 pm | Permalink

      Ben is right, IIRC it is now believed ~ 10 % of comets were transferred between Oort clouds in the early days, when the Sun and its siblings formed close.

      Later some material can transfer too, due to gravitational tugs from passing systems.

      • Torbjörn Larsson, OM
        Posted November 19, 2014 at 4:01 pm | Permalink

        Oh, and about seeing them: the initial transfers were of much alike material, originally out of the same molecular cloud at the same local volume. So not easy to distinguish.

        • Posted November 19, 2014 at 4:06 pm | Permalink

          Reminds me, and you’d be the one to know…we have, haven’t we, identified at least a few of the stars that formed in the same stellar nursery as Sol, no?

          b&

          • Gerorge Martin
            Posted November 19, 2014 at 7:00 pm | Permalink

            I don’t know what Torbjörn knows, but I don’t think that they have. From an article that I read not too long ago (Scientific American?) they have a few possible candidates, but no one is sure yet.

            George

            • Torbjörn Larsson, OM
              Posted November 20, 2014 at 2:32 am | Permalink

              Morally correct, but just this spring an “almost certain” candidate was found:

              “Researchers have identified the first “sibling” of the Sun — a star that was almost certainly born from the same cloud of gas and dust as our star.”

              “The team identified HD 162826 as the Sun’s sibling by following up on 30 possible candidates found by several groups around the world looking for solar siblings.”

              “Combining information on both chemical makeup and dynamics of the candidates narrowed the field down to one: HD 162826.”

              [ http://www.astronomy.com/news/2014/05/astronomers-find-suns-long-lost-brother-pave-way-for-family-reunion ]

              • Posted November 21, 2014 at 9:55 am | Permalink

                Thanks! I’m pretty sure that’s the news I had in mind.

                Frankly, I have a difficult time imagining that none of our few hundred stellar littermates have life….

                b&

            • Torbjörn Larsson, OM
              Posted November 20, 2014 at 2:33 am | Permalink

              And the estimates of the number of siblings is ~300 – 600 AFAIK, depending on the models. A needle in the Milky Way haystack.

  18. Torbjörn Larsson, OM
    Posted November 19, 2014 at 3:11 pm | Permalink

    That is a good question. Unfortunately it has a long answer.

    Twenty years ago, when Rosetta was conceived, there was a lot less known about planet formation and early evolution.

    Comets was thought to solve a number of problems, and as Mark notes it is not necessarily bad to remind people of these opportunities:

    1. The problem of volatile delivery. At the time it was thought that the disk was orderly (or if not, there wasn’t much evidence to the contrary) and planet formation took a long time. Hence it was thought that the majority of water, carbon and nitrogen could have been delivered by comets.

    2. The dilution problem. Since the main theory was (and perhaps remain) “soup theories” it was thought by Sagan et al that organics had to be delivered en masse. They could be cranked by Miller’s heat, discharges and UV to produce enough organics in the then presumed acidic atmosphere.

    3. The chirality problem. It was thought that a large enough chiral breaking between molecules would have to be seeded before amplification could drive it to 100 %. Meteorites was observed to supply a slight left-handed excess of amino acids. It was assumed comets could do too, since such excesses could have been driven by a star, which then rotating may radiate non-circular polarized light in certain directions.

    4. The time problem. Life was tentatively seen right after the end of the heavy late bombardment. It was assumed the latter was unsurvivable, in which case cometary panspermia would have solved the problem.

    All these problems have been solved or are promised to be solved without comets. (See below for a short rundown.) Meanwhile heavy constraints that could bar comets as major deliverers have been presented.

    My hope is in fact that Ptolemy, which apparently run on the dust thrown up at Philae’s first bounce as it was believed the craft was landed, can help ground truth (as best dust and coma can do) the spectroscopic nitrogen isotope ratios. They exclude comets from delivering much of any water. But note that D/H ratios alone can’t do that, some comets have the same D/H as Earth, Moon, Vesta, Mars. [“Early accretion of water in the inner solar system from a carbonaceous chondrite-like source”, Sarafian et al, Science, Oct 2014]

    Already before that, the discovery that perhaps up to half the volatiles were delivered to the disk from the molecular cloud that produced it, cut the comet fraction from at most 40 % to 20 % and more likely a few %. That is consistent with the above find.

    Solutions:

    1. Volatile problem:

    Just this week a first observation of the early disk magnetic field has been presented, from chondrule magnetic grains. Result: The early disk had strong local fields that could brake and swirl the disk, making it clear into the Sun in short order. And it puts the grains as most likely shocked by the early star’s antics.

    Shocks and swirls would have promoted fast planetesimal formation, and water delivery with some of them. This is consistent with planetary system formation models that has become popular to probe these ideas.

    2. Dilution problem:

    Alkaline hydrothermal vent, or “battery” redox theories, solves both dilution and organics production in a near neutral CO2 dominant atmosphere.

    3. Chirality problem:

    Pathways in 2 are non-chiral. Cross-chiral polymerase ribozymes can support such metabolism up until the evolution of the genetic code. [See my longish comment under the “RNA world” article for reference. Another result hot from the presses!]

    4. Time problem:

    The heavy late bombardment has been shown to be survivable in models. Prokaryotes proliferate and spread faster than the sterilizing impactors can keep up.

    And the oldest 4.4 Ga bp Jack Hill zircon has delivered robust oxygen isotope ratios after 6 [!] years microanalysis to find the undamaged clusters in the crystal. The early Earth was “cold” and wet.

  19. conn suits
    Posted November 19, 2014 at 6:37 pm | Permalink

    1) Thank you. Splendid.
    2) SULFUR! I just felt that should be added 😉
    3) Comets with proteins on them has incredible schlocky movie possibilities! The proteins, nonsentient replicators, would rapidly recruit the plants in a war against humans. Then they’d call in the carbon dioxide comets to provide abundant food for their new allies. A whole new genre of silly comet movies could be born from the current hype!

    • Torbjörn Larsson, OM
      Posted November 20, 2014 at 2:27 am | Permalink

      Sulfur, yes. Murchison shows that the CHON compounds are produced first, then S is substituted into them. I see above that they have found the last CHNOPS element P that is major part of cells too, phosphates in some asteroids (if not Murchison).


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